The present invention relates to electric motors. More specifically, the invention relates to a method and apparatus for operating ac motors.
A typical oil refinery has a large number of pumps that are driven by electric motors. Electrical power is typically distributed to the electric motors via a power grid. The power grid, in turn, receives electrical power from a remote utility.
Certain drawbacks are associated with distributing electrical power to the motors via such a power grid. For instance, the power grid can be expensive to establish, especially for a large refinery. Additionally, transmission losses can occur across the grid while electrical power is being distributed to the various motors. Transmission losses can also occur while electrical power is being transmitted to the power grid from the remote utility. Moreover, distributing the electrical power can be unreliable.
Certain problems are also associated with the electric motors. Load conditions on the electric motor often vary during normal operation. An electric motor that is operated at a constant speed will operate efficiently under full load conditions, but it will operate inefficiently under part load conditions. Thus, inefficient operation due to variable load operation can pose a problem.
Another problem can occur during startup of the electric motor. During startup, the motor receives an inrush of current. The inrush current is typically four to six times the current received during steady state operation. Consequently, motor power rating is constrained between one-quarter to one-sixth of the maximum power output of the power grid.
The problems arising from variable load conditions and inrush current may be overcome by the use of a variable speed drive. The variable speed drive allows the electric motor to operate more efficiently under part load conditions. The variable speed drive also limits the inrush of current during startup. However, variable speed drives are typically expensive. Additionally, variable frequency drives have internal losses associated with their own operation.
Thus, there is a need to limit inrush current during startup and increase efficiency of the electric motor during normal operation, without the use of a variable speed drive. There is also a need to increase energy savings and improve the reliability of distributing electrical power to the electric motors.